1. Field of the Invention
The present invention relates to an electrophotographic photocanductor for use in a printer, a copying machine, and a facsimile machine, and in particular to an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer formed thereon, with the top surface layer of the photoconductor containing a specific high-molecular charge transport material to minimize the wear of the photoconductor and improve the chemical stability to oxidized material generated in the above-mentioned electrophotographic apparatus.
2. Discussion of Background
According to the electrophotographic image forming method employing the Carlson process, the surface of a photoconductor is uniformly charged, and exposed to light images, thereby forming latent electrostatic images on the surface of the photoconductor. The latent electrostatic images thus formed are developed to visible toner images with a toner, and the toner images are transferred to an image receiving member such as a sheet of paper, and fixed thereon. After the toner images are transferred to the image receiving member, residual toner remaining on the surface of the photoconductor is removed therefrom, and the photoconductor is subjected to quenching step. Thus, the photoconductor can be repeatedly used for an extended period of time.
The electrophotographic photoconductor is required to have basic electrophotographic properties such as stable charging characteristics and good sensitivity, and minimal dark decay. In addition to the above, the photoconductor is further required to have satisfactory physical properties from the viewpoints of printing resistance, wear resistance, and moisture resistance. Further, it is important for the photoconductor to exhibit an environmental resistance, to be more specific, the resistance to ozone generated in the course of corona charging, and the resistance to ultraviolet light in the course of light exposure.
In recent years, for achieving color image formation, a plurality of photoconductors is incorporated in an electrophotographic machine such as a copying machine, printer, and facsimile machine. Further, in line with the trend toward small-size apparatus, there is a tendency to decrease the diameter of the photoconductor. However, the smaller the diameter of the photoconductor, the more conspicuous the abrasion of the photoconductor even though the printing operation is repeated the same number of times. The result is that the charging characteristics of the photoconductor are decreased and the life of the photoconductor is thus curtailed. It is conventionally inevitable to replace such a photoconductor or a developing unit including the photoconductor that is no longer used. To extend the life of the photoconductor itself as long as possible is desirable from the environmental viewpoint.
One of the typical examples of the photoconductors which are put in practical use is an organic photoconductor having such a layered structure that a charge generation layer (CGL) and a charge transport layer (CTL) are successively overlaid on an electroconductive support.
The charge transport layer (CTL) comprises a binder resin and a low-molecular charge transport material (CTM). In this case, the low-molecular charge transport material is usually contained in the charge transport layer in an amount of as large as 40 to 50 wt. % in order to exhibit sufficient charge transporting properties. The addition of such a large amount of low-molecular charge transport material (CTM) lowers the intrinsic mechanical strength of the binder resin employed for the charge transport layer. Such lowering of the mechanical strength of the charge transport layer will decrease the wear resistance of the obtained photoconductor.
To solve the above-mentioned problem, a variety of binder resins with excellent mechanical strength have been developed. However, the amount of the low-molecular charge transport material contained in the charge transport layer is so large that the binder resin cannot exhibit its own properties sufficiently.
It is thus proposed to use a high-molecular charge transport material with excellent mechanical characteristics instead of the aforementioned low-molecular charge transport material.
Some vinyl polymers such as polyvinyl anthracene, polyvinyl pyrene and poly-N-vinylcarbazole have been studied as the high-molecular charge transport materials for the organic photoconductor of a charge transport complex type. However, such vinyl polymers are not satisfactory from the viewpoint of photosensitivity.
In addition, high-molecular materials with charge transporting properties have been also studied to eliminate the shortcomings of the above-mentioned layered photoconductor. For instance, there are proposed an acrylic resin having a triphenylamine structure as reported by M. Stolka et al., in "J. Polym. Sci., vol 21, 969 (1983)"; a vinyl polymer having a hydrazone structure as described in "Japan Hard Copy '89 p. 67"; and polycarbonate resins having a triarylamine structure as disclosed in U.S. Pat. Nos. 4,801,517, 4,806,443, 4,806,444, 4,937,165, 4,959,288, 5,030,532, 5,034,296, and 5,080,989, and Japanese Laid-Open Patent Applications Nos. 64-9964, 3-221522, 2-304456, 4-11627, 4-175337, 4-18371, 4-31404, and 4-133065. However, any of these materials have not yet been put to practical use because of the low sensitivity or high residual potential.
The development of high-molecular charge transport materials has been proceeding. Some are found to provide a photoconductor with a sufficient sensitivity and a low residual potential as disclosed in Japanese Laid-Open Patent Applications 7-325409, 9-127713, 9-297419, 9-80783, 9-80784, 9-80172, 9-222740, 9-265197, 9-295201, 9-211877, 9-304956, 9-304957, and 9-329907. The wear resistance of these photoconductors is improved when compared with that of the conventional photoconductors, so that the life of the photoconductors can be extended to some extent.
However, another factor to shorten the life of the photoconductor is the deterioration of charging characteristics resulting from the exposure of the photoconductor to ozone and NOx gases. Most of electrophotographic image forming apparatus employ a corona charger at the charging step. In the course of the charging step, ozone gas generated from the corona charger and nitrogen compound gas (NOx) synthesized from the ozone and nitrogen in the air are brought into contact with the surface of the photoconductor, thereby causing the charging characteristics of the photoconductor to deteriorate during the repeated use. To eliminate the above-mentioned problem, the contact charging method, for example, using a charging roller currently becomes prevalent instead of the corona charging method for reducing the generation of oxidized gases. Although the amount of ozone and the reactive gases can be reduced by the contact charging method, the problem of adverse effect on the photoconductor by such reactive gases has not yet been solved. The decrease in charging characteristics due to the exposure of the photoconductor to the reactive gases will become a significant factor to determine the life of the photoconductor as the wear resistance of the photoconductor is increasing. In other words, there remains the problem that the life of the photoconductor cannot be extended in spite of the improvement in wear resistance because insufficient gas resistance lowers the charging characteristics.
To improve the resistance of the photoconductor to such oxidized gases, addition of a hindered phenol or p-phenylenediamine, which is known as an antioxidant for the conventional photoconductors, is disclosed in Japanese Laid-Open Patent Applications 63-18356, 63-50849, and 63-44662. Such a method of adding the antioxidant to the photoconductor can be applied to the previously mentioned organic photoconductor comprising a high-molecular charge transport material. In this case, however, it is preferable to minimize the amount of antioxidant. This is because the wear resistance of the photoconductor is lowered by the addition of a large amount of antioxidant. In view of the above-mentioned fact, it is considered that when the high-molecular charge transport material itself has sufficient gas resistance, the best results can be produced.
However, no attention has been paid to the gas resistance of the employed high-molecular charge transport material in the development of photoconductors employing the high-molecular charge transport materials.
Japanese Laid-Open Patent Application 9-297419 discloses a photoconductor which comprises a polycarbonate copolymer resin having an aromatic tertiary amine structure or a triarylamine structure, with the aim to improve the wear resistance of the photoconductor. No consideration is given to the gas resistance of the photoconductor in this application, so that the life of the photoconductor cannot be extended in terms of the gas resistance.